Membranes Based on PPO and Various Molecular Sieves for Syngas Purification

In this work, we verified the feasibility of H₂-selective MMMs based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and two different molecular sieves; the hydrophobic Metal Organic Framework, ZIF-8 and hydrophilic Zeolite 3A, for the purification of syngas obtained from gasification. Pre-combustion separation is of interest for membrane technologies because of the favorable operating conditions, such as a high pressure and driving force (Marano et al. 2009).
Membranes containing ZIF-8 showed good compatibility and weight fractions as high as 45% could be obtained without defects. DSC and TGA measurements indicate good adhesion between the two phases. The permeability of He, H₂, CO₂, N₂ and CH₄ is extraordinarily increased by the addition of ZIF-8, up to +800%, and the selectivity is also enhanced.
Permeability and selectivity of PPO-ZIF 8 membranes were compared to various models like the Maxwell- Wagner-Sillars, and a percolation treshold was identified at a weight fraction of ZIF-8 of about 15-20%, which accounts for the more-than-ideal increase of permeability measured. Data at temperatures as high as 65°C also allowed to estimate the activation energies. Finally, density and gas sorption tests on the various membranes allowed to verify the validity of the additive model for estimating the properties of mixed matrix membranes from the corresponding pure component ones.

A similarly comprehensive experimental analysis was carried out on membranes formed by PPO and a highly selective inorganic molecular sieve, Zeolite 3A, at loadings as high as 25 wt%. Addition of this hydrophilic filler leads to a decrease of permeability at room temperature, due to the obstruction of zeolite pores by the moisture absorbed from the environment, as it was verified with DSC measurements. Such effect eventually disappears at higher temperatures. The compatibility, and water sensitivity of the Zeolite was improved by silane-modification of its surface, thus obtaining composite membranes with a good homogeneity. Also, the gas permeability of the PPO-Zeolite membranes modified in this way is improved, as well as the selectivity, indicating that the hydrophobization of the surface enables the selective ability of the zeolite pores.

Both types of composite membranes show high thermal stability and their performance surpasses the Robeson's H₂/CO₂ upper bound at temperatures of about 180°C.